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tained at approximately 80 C by a heating tape and were subsequently calcined
at 400 C for 4 h to remove the surfactant [16]. Mesoporous silica microparticles
thus obtained were modified by surface grafting according to the procedure described by Huang and Wirth [22] adapted to N-isopropylacrylamide (NIPAAm). Hydroxyl groups were created on the silica surface by treatment with
concentrated HNO3 for 4 h and subsequent washing with ultra-pure (> 18 MX
resistance) water and then drying at 110 C for 2 h under an N2 stream. These
particles (0.5 g) were then added to a reactor containing 0.5 mL of the initiator,
1-(trichlorosilyl)-2-(m/p-(chloromethyl) phenyl)ethane (Gelest), and 50 mL of
anhydrous toluene. The reaction was carried out at room temperature for 12 h.
The silica particles were then washed with toluene, methanol, and acetone and
dried at 110 C for 2 h. Atom transfer radical polymerization (ATRP) was performed on the initiator-derivatized particles. 0.2 g of silica particles were combined with 0.107 g CuCl, 0.5 g of bipyridine, and 3 g of NIPAAm (Aldrich) in
30 mL of dimethyl formamide. The reaction flask was deoxygenated with N2
for 40 min and then sealed under N2. The reaction took place at 130 C for 40 h
with stirring. The grafted particles were then washed with methanol and water
and dried at 70 C under a stream of N2.
The particles were characterized by SEM (Hitachi S-800) and X-ray diffraction (Siemens D5000, Cu Ka radiation k = 1.5418 Š). Particle sizes were measured from the SEM images. Surface area and pore size distribution studies
were carried out using nitrogen adsorption and desorption at 77 K using a Micromeritics ASAP 2000 porosimeter. Sample preparation for cross-sectional
transmission electron microscopy (JEOL 2010, 200 KV) required the particles
to be embedded in an epoxy and then cross-sectioned using a Sorvall MT-5000
Ultra Microtome machine.
Polymer-modified particles (1.0 mg) were added to 1.0 mL of 0.035 mM
fluorescein in Tris buffer (0.05 M, pH 7.4) and were incubated at either 25 or
50 C for 2 h. The samples were then cooled down to room temperature
(~ 25C), and equilibrated at this temperature for at least 40 min. The particles
were then washed three times in fresh Tris buffer. Flow cytometry recorded the
dye remaining in the particles at both 50 C and 25 C. The uptake of dye was
also measured, using flow cytometry, at various temperatures to examine the
temperature response of the grafted polymer.
Bead suspensions were analyzed by flow cytometry using a Becton-Dickinson FACScan flow cytometer (Sunnyvale, CA) interfaced to a Power PC Macintosh using the Cell Quest software package. The FACScan is equipped with a
15 mW air-cooled argon ion laser. The laser output wavelength is fixed at
488 nm. Experimental details of these analyses have been described elsewhere
[23,24].
For confocal microscopy, 5.0 mg of polymer grafted particles were incubated
in 0.4 mL of an aqueous solution containing 0.5 mM of rhodamine 6G (Molecular Probes) at either 25 or 50 C overnight. The samples were then cooled to
room temperature (~ 25C), and equilibrated at this temperature for at least
40 min. The particles were washed two times with water at room temperature.
The dye remaining in the particles was observed at 50 C or 25 C using a confocal laser scanning microscope at regular intervals. Microscopy was performed
on a Zeiss Axiovert microscope using an LSM 510 (Carl Zeiss) scan head. Simultaneous DIC imaging was performed using the scan head and LSM software
and a Plan Neo Fluor 40X/1.3 NA oil immersion lens. A He±Ne laser (543 nm)
was used to excite fluorescence. Samples were heated by placing a suspension
of particles on a slide supported on a heating stage constructed from an aluminum block and a self-adhesive heating element. The temperature of the sample
was probed using a thermocouple.
±
Received: March 26, 2003
Final version: May 2, 2003
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[13] A. Kikuchi, T. Okano, Adv. Drug Delivery Rev. 2002, 54, 53.
[14] A. S. Hoffman, Clin. Chem. 2000, 46, 1478.
[15] S. Balamurugan, S. Mendez, S. S. Balamurugan, M. J. O'Brien, G. P. Lopez, Langmuir 2003, 19, 2545.
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Brinker, Nature 1998, 394, 256.
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Self-Assembly of Mesostructured Conjugated
Poly(2,5-thienylene ethynylene)/Silica
Nanocomposites**
By Byron McCaughey, Chris Costello, Donghai Wang,
J. Eric Hampsey, Zhenzhong Yang, Chaojun Li,
C. Jeffrey Brinker, and Yunfeng Lu*
Nanocomposites, consisting of intricate combinations of organic, inorganic, and/or metallic phases arranged on the nanometer scale, have attracted a great deal of attention due to
their unique ability to combine and tailor overall physical
properties and due to synergy, interfacial, and confinement effects between the constituent phases.[1] Nanocomposites that
contain conjugated polymers such as poly(phenylene vinylene),[2] polyaniline,[3,4] polydiacetylene,[5] poly(phenylene butadiynylene),[6] polythiophene, polypyrrole, and polyacetylene[7,8] confined within a silica matrix have shown enhanced
conductivity, mechanical strength, processability, environmen-
±
[*] Prof. Y. Lu, B. McCaughey, D. Wang, J. E. Hampsey
Chemical Engineering Department, Tulane University
300 Lindy Boggs, New Orleans, LA 70118 (USA)
E-mail: [email protected]
Dr. C. Costello, Prof. C. Li
Chemistry Department, Tulane University
2015 Percival Stern, New Orleans, LA 70118 (USA)
Dr. Z. Yang
State Key Laboratory of Polymer Physics and Chemistry
The Chinese Academy of Sciences
Beijing 100080 (P.R. China)
Prof. C. J. Brinker
Advanced Materials Laboratories, University of New Mexico
1001 University Boulevard SE, Suite 100, Albuquerque, NM 87106 (USA)
[**] The authors thank Dr. J. Tang, Dr. W. Zhou, Windong Chen at University
of New Orleans Tulane University for assistance with XRD and TEM.
This research was supported by the NSF/EPA joint program under grant
NSF-DMR-0124765 and Sandia National Laboratories.
DOI: 10.1002/adma.200304931
Adv. Mater. 2003, 15, No. 15, August 5
Adv. Mater. 2003, 15, No. 15, August 5
http://www.advmat.de
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tal stability, and other unique properties that allow for potential use in light emitting diodes,[9] information storage devices,
optical signal processors, battery substitutes, and solar energy
converters.[10] Areneethynylene conjugated polymers such as
poly(2,5-thienylene ethynylene) (PTE) are particularly interesting because they have exhibited high quantum yields,[11]
lower bandgaps than benzene analogs,[12] and stability to air
and moisture.[13]
Conjugated polymer/ceramic nanocomposites are typically
synthesized via a multiple-step synthesis process involving the
formation of mesoporous material and subsequent monomer
infiltration and polymerization;[2±4,6±8] however, polymerizations of conjugated polymers like polyacetylene often require
strict synthesis conditions.[10] Recently, we developed a robust,
one-pot polymer synthesis procedure to prepare PTE via the
palladium-catalyzed polymerization of aryl diodides with
acetylene gas in aqueous media.[14] The use of water as the reacting medium enables the use of the surfactant self-assembly
synthesis route[15] to produce mesostructured conjugated
polymers or composites.
This research utilizes the evaporation-induced co-assembly
of silica, surfactant, and organic components in a mixed alcohol/water medium to form mesostructured PTE/silica nanocomposites.[16] Beginning with a homogenous solution of soluble silica, hydrophobic catalytic species, 2,5-diiodothiophene
polymer precursors, and surfactant in ethanol/water solvent;
solvent evaporation during the coating process enriches nonvolatile components and induces their co-assembly into liquid
crystalline mesophases. Polymerization of silica during the
coating process freezes the mesophases and spatially organizes the catalyst and polymer precursor into hexagonal,
cubic, or lamellar silica mesostructures.[17,18] The subsequent
Sonogashira coupling reaction of acetylene with the incorporated 2,5-diiodothiophene and Pd catalyst[19] forms a PTE/silica nanocomposite that retains the original liquid-crystalline
mesostructure. As opposed to the commonly utilized multistep infiltration techniques, this one-step approach provides a
simple route to spin-coat, dip-coat, or inkjet print thin films
and patterned arrays of mechanically stable conjugated polymer/silica nanocomposites.[17,18,20]
X-ray diffraction (XRD) performed on a series of spincoated samples outlined the three successive stages of nanocomposite formation. Figure 1 shows XRD spectra of mesostructured films A) without the incorporation of monomer or
catalyst, B) with the incorporation of monomer and catalyst
but prior to polymerization, and C) after polymerization. Diffraction peaks at 45, 49, and 53 Š d-spacing in scan (A) are
respectively attributed to cubic mesophase [211], [210], and
[200] orientations.[21] As shown in the inset to Figure 1, these
diffraction peaks shift to higher d-spacing due to the incorporation of monomer/catalyst (scan B) and to the further incorporation and reaction of acetylene gas that forms the PTE
(scan C) within the hydrophobic core of the liquid-crystalline
mesophase.
Figure 2 shows representative transmission electron microscopy (TEM) images of the polymer/silica nanocomposites.
Fig. 1. XRD of mesostructured silica/surfactant thin films: A) without monomer or catalyst; B) with catalyst and monomer incorporation but prior to polymerization; C) after polymerization yielding a PTE/silica nanocomposite. Inset
table shows peak positions in angstroms.
Fig. 2. TEM images of PTE/silica nanocomposite with cubic mesostructure
viewed from A) [100] orientation and B) [111] orientation.
The highly ordered nanocomposite shows a center-to-center
spacing of A) 58 Š and B) 64 Š, which may be attributed to
the [100] and [111] orientations of a cubic mesophase, respectively. Shrinkage of the samples in the TEM chamber may
cause a decrease in these d-spacing values in comparison to
the XRD results shown in Figure 1C. Energy dispersive spectroscopy (EDS) elemental analysis identified silicon, oxygen,
carbon, and sulfur as expected for the PTE/silica nanocomposite. After silica removal, the porous nanostructure of the
poly(2,5-thienylene ethynylene) was observed by TEM; however, this structure was very sensitive to the electron beam
and no images could be recorded.
Polymer formation was further verified by UV-vis and
FTIR spectroscopy. Figure 3 shows the UV-vis spectra of thin
films A) without catalyst or monomer, B) with catalyst and
monomer but prior to polymerization, and C) after polymerization. A shoulder peak at 365 nm in (B) indicates the presence of Pd catalyst complex prior to polymerization. Polymerization initiated by exposing the catalyst±monomer complex
to acetylene gas results in a more intense absorbance especially above 300 nm (C), which is similar to those of areneethynylene polymers that show an absorbance between 350
and 600 nm.[22] FTIR spectroscopy on PTE polymer after silica and surfactant removal shows strong thiophene stretching
bands at 1602 and 1436 cm±1; out of plane thiophene C±H
bending bands at 1097 and 1004 cm±1; and low intensity ethynylene vibration around 2260 cm±1 due to the symmetry of the
ethynylene substitutes.
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Fig. 3. UV-vis spectra of thin films: A) without catalyst or monomer; B) with catalyst and monomer but prior to polymerization; C) after polymerization showing the presence of Pd catalyst complex and poly(2,5-thiophene ethynylene); D) prepared using different concentrations of catalyst and E) monomer (spectra after catalyst peak subtraction). Expected PTE structure shown in inset.
PTE chain rigidity[22] results in low solubility,[23] and a low
degree of polymerization.[24] Direct molecular weight determination is difficult. In this research, we attempted to study
the degree of polymerization using UV-vis spectroscopy.
Figure 3D,E shows UV-vis spectra of the nanocomposites prepared with different concentrations of catalyst (D) and monomer (E). Increasing the monomer concentration from 0.01 to
0.09 mol L±1 shifts the absorption band from 406 to 495 nm
with an increase in absorption intensity. An increase in catalyst concentration from 0.003 to 0.032 mol L±1 also increases
the absorption intensity but shifts the absorption from 469 to
431 nm. Oligomeric 2,5-thiophene ethynylene with 1±4 repeat
units shows UV-vis peaks from 317 to 400 nm,[12] while PTE
with an average molecular weight of 21 600 exhibits a UV absorbance that monotonically decreases from 200 nm to
600 nm.[13] Since polymer with longer chains may exhibit UVvis peaks at higher wavelengths, our results indicate that the
degree of polymerization increases as the monomer concentration increases or as the catalyst concentration decreases.
However, the exact molecular weight of our polymer is still
uncertain and further experimentation is needed.
Thermal stability of the nanocomposites was studied by simultaneous differential thermal analysis/thermogravimetric
analysis (DTA/TGA). Figure 4 shows the DTA/TGA curves
of A) pure mesostructured polymer after surfactant and silica
removal and B) as-reacted polymer/silica/surfactant nanocomposites. The small, sharp endothermic peak around 70 C
on curve B is due to the removal of residual solvent. The
weight losses of approximately 15 % (A) and 35 % (B) around
350 C are caused by the decomposition of short chain hydrocarbons such as oligomeric products, surfactant, and PPh3.
The exothermic peaks at 226 C may have been the evidence
of thermal cross-linking or polymer chain realignment.[25] In
both samples, polymer decomposition is indicated by a broad
endothermic peak accompanied by a gradual weight loss
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Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Fig. 4. Simultaneous DTA (bottom two lines)/TGA (top two lines) of A) pure
poly(2,5-thienylene ethynylene) after silica and surfactant removal and B) asreacted polymer/silica/surfactant nanocomposite.
above 600 C. For the composite (B), the remaining weight of
40±50 % at 1100 C corresponds to the initial carbon and silica
contained in the composite. On the other hand, the pure polymer (A) retains close to 75 % of its weight as residual carbon
at 1100 C, which is also consistent with its initial carbon composition (~ 68 %).
We have demonstrated the formation of conjugated PTE/
silica nanocomposites by a novel sol±gel self-assembly and an
air- and water-insensitive organometallic-catalyzed polymerization method. The robustness of this synthesis is further
demonstrated by the ability of the oligomeric PTE thin films
to continuously react with acetylene gas that results in darkened films with increased UV-vis absorptions. This approach
provides a unique and ready route to synthesize nanostructured conjugated polymer and composites. Future work will
focus on the studies of their mechanical, electrical, electroluminescent, and other properties of these nanocomposites with
various mesostructures and compositions.
http://www.advmat.de
Adv. Mater. 2003, 15, No. 15, August 5
Tetraethoxysilane (TEOS), nitric acid, surfactant, and 2,5-diiodothiophene
were mixed in tetrahydrofuran (THF) at room temperature for half an hour to
form a precursor solution. The catalytic complex was formed by reacting palladium acetate, cuprous iodide, and triphenylphosphine (PPh3) in THF. A typical
synthesis utilizes a molar ratio of reactants of THF/HNO3/TEOS/Brij-58/PPh3/
CuI/Pd(OAc)2/2,5-diiodothiophene of 6.2:0.356:0.937:0.1:0.04:0.014:0.017:0.1,
where Brij-58 is a non-ionic surfactant CH3(CH2)15(OCH2CH2)20OH. Films
were prepared by either spin-coating the mixture of the catalytic complex and
precursor solutions onto glass slides using a Specialty Coating Systems P-6000
spin-coater or allowing them to evaporate in a petri dish to form xerogels. The
thin films were exposed to acetylene gas and triethylamine vapor at room temperature and 50 psi pressure for up to 3 days to polymerize the incorporated
monomer. Pure conjugated PTE was obtained by washing with dilute HF to remove the silica and by ethanol extraction to remove the surfactant. The PTE
was washed with ethanol to remove the unbound catalyst and partially dissolved in chloroform to achieve oligomeric PTE solutions that were subsequently coated on glass slides to form red oligomeric PTE films. The oligomeric
PTE films were exposed to acetylene gas and the continuous reactions of acetylene with the active complex attached to the oligomers resulted in darkened
films with increased UV-vis absorptions.
The nanocomposites were characterized by transmission electron microscope
(TEM, JEOL 2010, operated at 200 kV), X-ray diffraction (XRD, Phillips
Xpert X-ray diffractometer using Cu Ka radiation at k = 0.1542 nm), UV-vis
spectroscopy (Beckman DU460B UV-vis), Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet Nexus 670 spectrophotometer with a Smart
MIRacle horizontal attenuated total reflectance Ge crystal accessory), and by
differential thermal analysis and thermogravimetric analysis (DTA/TGA, 2960
Simultaneous DTA±TGA by TA Instruments) operated at 10 C min±1 from 30
to 1200 C with argon sweep gas.
±
Received: January 28, 2003
Final version: April 10, 2003
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Adv. Mater. 2003, 15, No. 15, August 5
DOI: 10.1002/adma.200304669
Enhancement of Photocatalytic and
Electrochromic Properties of Electrochemically
Fabricated Mesoporous WO3 Thin Films**
By Sung-Hyeon Baeck, Kyoung-Shin Choi,
Thomas F. Jaramillo, Galen D. Stucky,
and Eric W. McFarland*
Tungsten oxide (WO3) is an indirect bandgap semiconductor with interesting photoconductive behavior that has potential applications as a low cost material for solar energy devices.[1,2] Because of its distinct photochromic response and
intercalation properties (H+, Li+, Na+, and K+),[3] it has also
found use in electrochromic devices and chemical sensors. For
the hydrogen cations, the electrochromic process of tungsten
oxide has been explained on the basis of the double intercalation of a proton and an electron to form a colored tungsten
bronze. WO3 has been prepared by physical and chemical
methods including sputtering and vacuum evaporation.[4]
Electrochemical synthesis has been accomplished by stabilizing tungsten ions in sulfuric acid[5] or as a peroxo complex.[6,7]
Because of the versatility of tungsten oxide in photo-electrochemical processes, the formation of high surface area, mesoporous tungsten oxide has also been of interest.[8±15] Synthesis
of porous oxides has followed early work on catalytic functional materials such as MCM-41 using surfactants and block
copolymers as templates.[16±21] Cheng et al.[9] reported enhanced electrochromic properties of sol±gel synthesized
mesoporous tungsten oxide; however, the material did not
exhibit long-range order of the pores. In general, synthesis of
well-ordered tungsten or molybdenum oxides has been limited by the formation of Keggin ions and difficulties in promoting their three-dimensional condensation to give stable
mesoporous structures.
We have recently described an electrochemical strategy for
the production of thin nanostructured films by utilizing potential-controlled self-assembly of surfactant±inorganic aggregates at solid±liquid interfaces.[22] This approach manipulates
surfactant±inorganic assemblies only in the thin interfacial
region by electrochemically controlling surface interactions,
which allows for the formation of nanostructured films from
dilute surfactant solutions. After deposition, surfactants can
±
[*]
Prof. E. W. McFarland, Dr. S.-H. Baeck, T. F. Jaramillo
Department of Chemical Engineering, University of California
Santa Barbara, CA 93106-5080 (USA)
E-mail: [email protected]
Prof. G. D. Stucky
Department of Chemistry and Biochemistry, University of California
Santa Barbara, CA 93106-5080 (USA)
K.-S. Choi
Department of Chemistry, Purdue University
West Lafayette, IN 47907-2084 (USA)
[**] Major funding was provided by the Hydrogen Program of the Department
of Energy (DOE, Grant # DER-FC36-01G011092). Partial funding and
facilities were provided by the NSF-MRSEC funded Materials Research
Laboratory (Award # DMR96-32716). Partial funding (GDS) was also
supported by the National Science Foundation (NSF Grant # DMR0233728).
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Experimental